Automatic range corrected flash ladar camera

ABSTRACT

A three dimensional imaging camera comprises a system controller, pulsed laser transmitter, receiving optics, an infrared focal plane array light detector, and an image processor. The described invention is capable of developing a complete 3-D scene from a single point of view. The 3-D imaging camera utilizes a pulsed laser transmitter capable of illuminating an entire scene with a single high power flash of light. The 3-D imaging camera employs a system controller to trigger a pulse of high intensity light from the pulsed laser transmitter, and counts the time from the start of the transmitter light pulse. The light reflected from the illuminated scene impinges on a receiving optics and is detected by a focal plane array optical detector. An image processor applies image enhancing algorithms to improve the image quality and develop object data for subjects in the field of view of the flash ladar imaging camera.

RELATED APPLICATIONS

This application claims priority to U.S. Provisional Application No.61/535,113 filed Sep. 15, 2011 which is incorporated herein byreference. Additionally, the subject matter disclosed in U.S. Pat. Nos.5,696,577, 6,133,989, 5,629,524, 6,414,746, 6,362,482, and U.S. PatentApplication Publication No. 2002/0117340 are incorporated herein byreference.

BACKGROUND

Conventional 2-D cameras for capturing visible images rely onrectangular arrays of pixellated light sensitive Charge Coupled Devices(CCDs) or CMOS sensors. These sensors are generically referred to asfocal plane arrays. They are positioned directly behind an imaging lenssystem in the focal plane of the lens. Similarly, the laser radardescribed herein relies on the performance of a focal plane arraydetector. The detector in this case must respond to wavelengths outsideof the visible range, in the near infrared spectrum. The detectormaterial is a binary compound of Indium Phosphide (InP), or a moreexotic quaternary compound such as Indium Gallium Arsenide Phosphide(InGaAsP) or a ternary combination of InGaP, InGaAs, or InAsP. A readoutintegrated circuit with an input amplifier is connected to each pixel ofthe focal plane array, to amplify and condition the laser radar returnsignals, enabling object detection and range determination. An internalclock and a timing circuit associated with each pixel in the infraredFocal Plane Array detector provide a range measurement for each pixel.The infrared focal plane array detector and readout integrated circuitinput amplifier together exhibit a time delay which varies with inputsignal amplitude, complicating the time of arrival determination for anyreflected light energy. To mitigate this effect, an optical sample ofthe transmitted laser pulse is fed back through a sacrificial pixel inthe infrared focal plane array and through the accompanying inputamplifier in the readout integrated circuit, producing a stable,amplitude adjusted zero time reference. Additionally, a precisiondelayed sample of the transmitted laser pulse is provided at the samesacrificial pixel of the infrared focal plane array, which allows forclock frequency calibration. These two performance features are referredto herein as Automatic Range Correction.

The wavelength of choice for the laser radar is typically 1570nanometers, which enables the invention in two respects: first, the 1570nm wavelength is inherently safer to the human eye, allowing for greaterpower transmission, and thus greater distance performance, and second,there are laser system components commercially available at thiswavelength which enable the design and production of a laser capable ofproducing the high energy pulse required. Other wavelengths may also besuitable for the laser radar and these other wavelengths may be utilizedwithin the scope of this invention.

In the design of a pulsed laser transmitter several limitations must beconsidered. The high peak power requirement necessitates the use of asolid state laser, typically employing a Neodymium-YAG laser rod, orErbium glass laser rod and a saturable Q-switch material, such asChromium doped Nd-YAG. Performance limitations of solid state lasersemploying these materials include the inability to accurately predictthe delay from a trigger pulse to the onset of a lasing pulse, modalstability, laser pulse shape, and laser pulse repeatability. Further,low power efficiency and thermal stability issues with the solid statematerials create additional challenges.

Therefore it is an object of this invention to provide a pulsed lasertransmitter with a higher efficiency, repeatable pulse shape, stabilityof wavelength, resistance to thermal stress, and a mechanism to overcomethe system limitations imposed by the unpredictable nature of the laserpulse startup delay, and to mitigate the effects of variable time delaysin the optical receiver focal plane array detector and readoutintegrated circuit.

FIELD OF THE INVENTION

The present invention relates to the field of remote sensing of objectsand the application of lasers to the problems of 3-D imaging, remoteobject detection and definition, terrain mapping, vehicle guidance, andcollision avoidance. Many attempts have been made to solve the problemof how to create the true 3-D imaging capability which wouldsimultaneously enable all of the applications cited. The instantinvention makes use of a number of new and innovative discoveries andcombinations of previously known technologies to realize the presentembodiments which enable the user to operate in a true 3-D mode withenhanced accuracy. This ability to operate the flash laser ranging anddetection device is provided by practicing the invention as describedherein. This invention relies on the performance of a multiple pixel,infrared laser radar for capturing three-dimensional images of objectsor scenes within the field of view with a single laser pulse, with highspatial and range resolution.

BACKGROUND

Many laser radar systems have been built which rely on the transmissionof a high energy illuminating pulse. Most often these systems rely onsolid state lasers operating in the near infrared with a lasing media ofNeodymium-YAG or Erbium doped glass. Many of these systems utilizemultiple pulses over a period of time to detect remote objects andimprove range accuracy. These systems are often based on a singledetector optical receiver. To develop a complete picture of a scene, thelaser and optical receiver must be scanned over the field of view,resulting in a shifting positional relationship between objects inmotion within the scene. Flash ladar systems overcome this performanceshortcoming by detecting the range to all objects in the scenesimultaneously upon the event of the flash of the illuminating laserpulse.

U.S. Pat. No. 6,392,747 awarded to Allen and McCormack and assigned toRaytheon, describes an improved scanning ladar, but makes the predictionregarding possible flash ladar systems, “an all digital implementationof a detector array is not possible.” U.S. Pat. No. 7,206,062 awarded toAsbrock, Dietrich, and Linder describes a readout integrated circuitadapted to process ladar return pulses by use of analog circuitry.

The present invention is a flash ladar camera system incorporatingelements of the flash ladar technology disclosed in Stettner et al, U.S.Pat. Nos. 5,696,577, 6,133,989, 5,629,524, 6,414,746B1, 6,362,482, andU.S. patent application US 2002/0117340 A1, and which provides with asingle pulse of light the range to every light reflecting pixel in thefield of view of the ladar camera as well as the intensity of thereflected light.

SUMMARY OF THE INVENTION

The present invention comprises a flash ladar camera system with anoptimized laser transmitter adapted to improve laser pulse shape andrepeatability, reduce pulse timing jitter, and provide an amplitudecorrected transmit timing reference. Additionally, the flash ladarcamera of the instant invention makes use of digital signal processingtechniques to improve reliability, repeatability, and thermal stability.The result of these improvements is an improved range accuracy of theflash ladar system.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a system block diagram of the flash ladar camera system. Theflash ladar camera system comprises a laser transmitter 1, camerareceive optics 2, a system controller 3, an infrared optical receiver 4,and an image processor 5;

FIG. 2 is a block diagram of the side pumped variant of the LaserTransmitter denoted by the number 1 in FIG. 1;

FIG. 2A is a mechanical layout showing items of interest of the sidepumped variant of the Laser Transmitter 1 shown in block diagram form inFIG. 2;

FIG. 2B is a diagram of illumination patterns produced by variations ofthe Diffuser denoted by the number 26 in FIG. 2;

FIG. 2C is a diagram of a gain guiding variant of the Q-switch denotedby the number 20 in FIG. 2;

FIG. 2D is a diagram showing mode suppressing features of the back facetmirror, and the Output Coupler denoted by the numbers 29 and 23,respectively, in FIG. 2;

FIG. 3 is a block diagram of the end pumped variant of the LaserTransmitter denoted by the number 1 in FIG. 1;

FIG. 3A is a mechanical layout showing items of interest of the endpumped Laser Transmitter 1 shown in block diagram form in FIG. 3;

FIG. 4 is a block diagram of the side pumped disc variant of the LaserTransmitter denoted by the number 1 in FIG. 1;

FIG. 4A is a mechanical layout showing items of interest of the sidepumped disc variant of the Laser Transmitter 1 shown in block diagramform in FIG. 4;

FIG. 5 is a block diagram of the infrared optical receiver denoted bythe number 4 in FIG. 1;

FIG. 6 shows details of the ARC signal and the delayed ARC signaldenoted by the numbers 69 and 71, respectively, in FIG. 5, and theircoupling to the input of the APD focal plane array denoted by the number73 in FIG. 5;

FIG. 6A shows an alternate embodiment of the ARC signal and the delayedARC signal denoted by the numbers 69 and 71, respectively, in FIG. 5,and their coupling to the input of the APD focal plane array denoted bythe number 73 in FIG. 5;

FIG. 6B shows another alternative embodiment of the ARC signal and thedelayed ARC signal denoted by the numbers 69 and 71, respectively, inFIG. 5, and their coupling to the input of the APD focal plane arraydenoted by the number 73 in FIG. 5;

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION

A preferred embodiment of the present invention, the Automatic RangeCorrected (ARC) Flash Ladar is depicted in block diagram form in FIG. 1.The system is designed to produce a 3-dimensional image using a pulsedlaser transmitter, an infrared focal plane array detector, and timingcircuits associated with each pixel of the array detector. The ARC FlashLadar is capable of producing range and intensity data for any object orscene within its field of view from a single pulse of Laser Transmitter1, in conjunction with System Controller 3, and Infrared OpticalReceiver 4. An optional Image Processor 5 produces object data andenhanced 3-D images. Receive Optics 2 are interchangeable conventionaltelescopic lenses adapted to a specific range and field of view andserve in the same manner as in an ordinary 2D still or motion camera.The shorthand terms “flash laser radar”, and “flash ladar” may be usedinterchangeably herein to refer to Laser Transmitter 1, Receive Optics2, System Controller 3, and Infrared Optical Receiver 4 collectively,and to their mutual operation.

Laser Transmitter 1 in FIG. 1 produces a high power, short durationlaser pulse illuminating the field of view of the Receive Optics 2. Thecommand to initiate the high power laser illuminating pulse is sent bythe System Controller 3 to the Laser Transmitter 1 via bidirectionalelectrical connections 9. Laser Transmitter 1 also provides a much lowerpower optical sample of the output illuminating pulse as an AutomaticRange Correction (ARC) signal to the Infrared Optical Receiver 4 viasignal connections 7, which include a fiber optic cable 28 as shown inFIG. 2. This lower power sample of the high power output laserilluminating pulse serves as a way to firmly establish the zero timereference for all time of flight measurements required for distanceestimation to any objects in the field of view of Receive Optics 2. Adelayed copy of the ARC signal serves as a reference for the accuratemeasurement of the System Controller 3 master clock frequency, providingan opportunity for the System Controller 3 to adjust and correct thereadout IC timing clock, or to calculate correction factors for thefrequency variance.

Receive Optics 2 has an optical aperture and a focal length fixed in oneposition by an application specific telephoto lens, or alternatively,may have a variable focal length lens with motorized drive. Subsequentto the flash of the pulsed Laser Transmitter 1, reflected light from thescene in the field of view of the Receive Optics 2 enters the lenssystem as Received Light signal 15. Once the Received Light signal 15 ispassed through the Receive Optics 2 lens system, it is properlyconditioned for input to the Infrared Optical Receiver 4. Thisconditioned optical input signal is passed to the Infrared OpticalReceiver 4 via optical connection pathway 12.

System Controller 3 is capable of operating either in a slave mode, oras master system controller. In a slave mode, the System Controller 3receives control commands from an external controlling computer ormaster console via Status/Control electrical connections 13 and alsoprovides camera status via these same bidirectional Status/Controlelectrical connections 13. As the master controller, System Controller 3is capable of running the camera independently, updating the camerasystem status and providing these status data to external monitoringequipment via bidirectional electrical connections 13.

System Controller 3 is notified by the Receive Optics 2 of the positionof its motorized zoom lens (ZOOM_STATUS) via bidirectional electricalconnections 6. System Controller 3 then may issue commands (ZOOM_CTRL)to the Receive Optics 2 to move the zoom lens to a chosen position viabidirectional electrical connections 6.

In the slave mode, the System Controller 3 issues commands and receivesstatus updates to Laser Transmitter 1, Receive Optics 2, InfraredOptical Receiver 4, and Image Processor 5 as described above, except thevalues for the commands are determined by an external computer or mastercontrol console.

Whether operating as a master controller or as a local controller slavedto an external master controller, System Controller 3 controls andinitiates the pulsing of the Laser Transmitter 1 by sending a TX_PULSEsignal via bidirectional electrical connection 9 to Laser Transmitter 1.The System Controller 3 also provides a system clock (SYS_CLK) to LaserTransmitter 1, Infrared Optical Receiver 4, and Image Processor 5 viabidirectional electrical connections 9, 10, and 11, respectively. Whenthe Laser Transmitter 1 sends a pulse downrange upon command from SystemController 3, it feeds back to the System Controller 3 via bidirectionalelectrical connection 9 a digital electrical signal (FLASH_DET) derivedfrom a sample of the Laser Transmitter 1 back facet mirror, which mirrorserves as the rear boundary of the Nd:YAG laser operating at thecharacteristic 1064 nm wavelength. This optical output is converted byoptical detector 31 in FIG. 2 into an electrical signal, and amplifiedand threshold tested against a preset value, giving an electrical logicsignal indicative of the initiation of a laser flash. This FLASH_DETsignal provides logic to turn off Diode Pump Lasers (18, 33 in FIG. 2)in the Laser Transmitter 1, and also gives an approximate zero timereference for the range counters which measure the range for anyreflected portion of the transmitted laser pulse incident upon any oneof the 16,384 pixels in a typical infrared focal plane array of InfraredOptical Receiver 4. System Controller 3 also provides camera statussignals to, and receives control commands from, a supervising mastercontroller or director via bidirectional electrical connections 13.

Once the initial flash of the Nd:YAG rod (19 in FIG. 2) has occurred,the front facet of the Q-switched pulsed laser is coupled to an OpticalParametric Oscillator (OPO denoted by the number 22 in FIG. 2), whichconverts the wavelength to the 1570 nm wavelength desired for sceneillumination. Laser Transmitter 1 also provides an optical sample of thepulsed laser output to the Infrared Optical Receiver 4 via an opticalcable (28 in FIG. 2). This optical sample from the front facet of thepulsed laser is provided by an optical pickup, and is the basis for theAutomatic Range Correction (ARC) capability of the improved flash ladarcamera. The ARC signal 28 may also be used to provide an improved zerotime reference with lower timing jitter than the FLASH_DET signalderived from the back facet of the Nd:YAG laser rod, which signal mayalso be used to turn off the Diode Pump Lasers 18 and 33, eliminatingthe need for the FLASH_DET back facet signal. There are significant, andsomewhat variable delays between the rising edge of the 1064 nm lightpulse of the basic Nd:YAG rod 19 and the rising edge of the OPO 22output at the transmission wavelength of 1570 nm. Use of the LaserTransmitter 1 front facet ARC signal 28 eliminates another source oftiming jitter in the establishment of the zero time reference, andtherefore improves the overall range accuracy of the flash ladar system.The ARC signal is also used to calibrate the readout IC timing clock,and is therefore critical in establishing and maintaining an accuratemeasurement of the elapsed time between transmit pulses and return lightpulses reflected from objects in the scene. Additional detail regardingthe functioning of the ARC signal is provided in the discussionassociated with FIG. 5.

Infrared Optical Receiver 4 incorporates an Avalanche Photodiode FocalPlane Array detector (73 in FIG. 5) positioned at the focal plane ofReceive Optics 2, and receiving light signals reflected from the sceneilluminated by the Laser Transmitter 1 pulsed output. A typical APDarray of the instant invention might have 128 rows of 128 individualpixel elements in a square arrangement. Each detector array element willhave connection to a low noise amplifier and a threshold detector set atan appropriate level to recognize the presence of an object orreflective surface in the illuminated scene. Each detector will alsohave a counter which initiates counting at the time of the laser flash,indicated electronically by the Laser Transmitter 1 ARC signal 28. Atthe instant the threshold detector for a given pixel exceeds the presetlevel, a logic signal will be produced, locking in the value of thecounter for the given pixel. In this manner, a range estimate can bemade based on the counter value for a selected pixel (RDAT). Theintensity data for the selected pixel is also stored as a series ofanalog samples associated with the selected pixel (IDAT). The SystemController 3 distributes a clock signal through bidirectional electricalconnections 10 to the Infrared Optical Receiver 4 which is used by thecounters associated with each pixel in the APD focal plane array todetermine range. Alternatively, the readout IC (74, 76 in FIG. 5) mayhave an onboard oscillator, enabling it to measure range dataindependently of the system clock distributed by the System Controller3.

System Controller 3 receives a ready signal (IPRO_RDY) from ImageProcessor 5 via bidirectional electrical connection 11 when it is hascompleted processing of the previous frame of video or still picture.The System Controller then issues a 3D_CONVERT command to ImageProcessor 5 when the Infrared Optical Receiver 4 outputs are ready.Image Processor 5 then receives raw range and intensity data (RDAT, DAT)from Infrared Optical Receiver 4 of the flash laser radar via electricalconnections 14. Image Processor 5 uses various mathematical and logicalalgorithms to adjust and correct the raw range and intensity data. Thiscorrected data may then be processed to produce object data in the formof vectors describing regular geometric shapes. Object Data is outputthrough electrical connection 8. The corrected range and intensity datamay also be output in the form of a series of still images, or as amoving picture (3D_Composite) via electrical connection 16.

FIG. 2 is a block diagram of the laser transmitter of a preferredembodiment denoted by the number 1 in FIG. 1. Shown is a high powerdiode pump laser 18 positioned on the long side of a solid state rodlaser gain media 19. The high power laser light from Diode Pump Laser 18is coupled through a companion rod lens (39 in FIG. 2A) to focus thelight interior to Laser Gain Media 19. Positioned on an opposing side ofthe rectangular rod shaped Laser Gain Media 19 is another high powerdiode pump laser 33 coupled through a second companion rod lens (40 inFIG. 2A) to Laser Gain Media 19. Laser Transmitter Controller 35receives a TX_PULSE command from System Controller 3 via bidirectionalelectrical connection 9 and applies large current pulses to both DiodePump Lasers (L) 18 and (R) 33, which lase at 808 nm, via electricalconnections 34. Laser Gain media 19 is Nd:YAG which absorbs photons at808 nm from the left (L) and right (R) Diode Pump Lasers 18 and 33. Theenergy level of the laser gain media 19 continues rising until Q-Switch20 starts to bleach. When Q-Switch 20 bleaches sufficiently, the singlepass gain along optical axis OA 25 surpasses unity, and a lasing modequickly builds up between the interior facet mirror of Output Coupler 23and the back facet mirror 29. The front facet mirror 21 is typically 99%transmissive and the back facet mirror 29 is typically less than 1%transmissive at the 1064 nm lasing wavelength of Laser Gain Media 19.Front facet mirror 21 serves to contain the Diode Pump Laser 18 and 33output light at 808 nm to the Laser Gain Media 19. Both back facetmirror 29 and front facet mirror 21 are greater than 99% reflective at808 nm. Laser Gain Media 19 is split into two roughly equal halves, withan internal Brewster angle window (41 in FIG. 3) which serves to coupleany horizontally polarized light out of the Laser Gain Media 19. Thisremoval of horizontally polarized light conditions the laser pulseappropriately for the frequency conversion process in Optical ParametricOscillator 22.

Laser light at 1064 nm exiting through the front facet mirror 21 ofLaser Gain Media 19 is incident upon Optical Parametric Oscillator 22, anon-linear doped KTP (Potassium Titanium Oxide Phosphate) crystal whichserves to convert the incident 1064 nm laser radiation into a 1570 nmlight pulse directed along optical axis OA 25. Front facet mirror 21 isalso greater than 99% reflective at 1570 nm, and serves to confine thefrequency converted output of the OPO between Front Facet mirror 21 andthe Output Coupler 23 exterior mirror surface. Output Coupler 23exterior facet mirror surface is typically 70% reflective at the nominal1570 nm lasing wavelength of Laser gain Media 19. The Output Coupler 23interior facet mirror surface stops the transmission of the shorterwavelength 1064 nm light and serves as the resonator mirror for the 1064nm lasing mode. The front, or exterior surface mirror of Output Coupler23 passes approximately 30% of the light at the converted wavelengthnear 1570 nm as the output pulse of Laser Transmitter 1. As noted, theexterior surface mirror of Output Coupler 23 is typically 70%reflective, which provides enough reflected 1570 nm light to the OPO 22to provide for efficient wavelength conversion in the OPO 22. Lightpulses exiting the front surface of Output Coupler 23 along optical axisOA 25 are incident upon Diffuser 26, which diffuses the high intensitylaser radiation into a square, top-hat shaped output beam designed touniformly illuminate the entire scene in the field of view of ReceiveOptics 2 along the optical axis 25. Diffuser 26 is a diffractive optic,commonly referred to in the literature as a holographic diffuser, and isdesigned to project the Laser Transmitter 1 output beam in the desiredpattern. Diffuser 26 may also be a conventional refractive lens element.Receive Optics 2 is positioned in a slightly offset position from LaserTransmitter 1 and boresighted in the same direction along Optical AxisOA 25 and held in position by the flash ladar camera body.

Other materials may be substituted for Nd:YAG as the Laser Gain Media 19such as Yb:YAG, Yb:Er:YAG, Er:Glass, etc. while maintaining theadvantages of the instant invention. Other rod geometry, such ascircular, elliptical, or hexagonal, may be used for the Laser Gain Media19 and associated elements of Laser Transmitter 1. Other crystals may beused for the OPO 22 such as KTiOAsO₄ (KTA) in order to improveperformance, or generate other wavelengths more suited to flash ladaroperation in underwater environments, or in the presence of fog, dust,or other atmospheric obscurants. Additionally, other adaptations ofDiffuser 26 may be beneficial to effect other than a square “top hat”output beam. It may be desirable to shape the Diffuser 26 to produce anoutput beam with a power density greater in the center than at thesides, to enhance distance viewing in the center of the field of view,or to direct the main focus of the beam energy at a desired angle. Forsuch applications, a non-uniform beam profile, or otherwise shaped beamoutput may be produced by an aspherically lensed Diffuser 26. These andother adaptations of the preferred embodiment described herein may bepracticed without changing the nature or effect of the instantinvention, or the associated claims.

FIG. 2B shows a number of illustrative beam profiles which may beeffected by Diffuser 26. Shown at the top left of FIG. 2B is aLambertian, or uniform optical power density profile 96 covering a full180°. The plot is shown in the horizontal plane, or the ⊖-plane, inspherical coordinates. This profile can be achieved using ahemispherically shaped lens as a Diffuser 26. At the bottom left of FIG.2B, a truncated Lambertian pattern is shown, which describes a uniformoptical power density pattern 99 which is limited by first limitingangle 97 at −60° and second limiting angle 98 at +60° in the exampleshown, for a total angular coverage of 120°. At the top right of FIG. 2Ba truncated and rotated Lambertian pattern 101 is shown which providesuniform optical power density over a 90° field of view, which has beenrotated 45° by rotation angle 100 in the example shown.

Finally, at lower right of FIG. 2B is shown a combination pattern termedForward Moving illumination pattern 105, which is conditioned forapplication to moving vehicles. Of primary concern to an automobile,airplane, train, trolley, robotic crawler, or other moving platform isthe need to avoid collisions with stationary or moving objects directlyin the path of the moving vehicle, but it is also necessary to have someability to sense nearby objects close to the operational path of thevehicle if the vehicle is turning or maneuvering. The need therefore, isto sense objects directly in the path of the vehicle at a greaterdistance, and to project a greater power in the narrow path directly inthe path of the vehicle, and project a reduced power pattern lateral tothe path of the vehicle to sense nearby objects which may interfere withthe lateral progress of the vehicle. The Forward Moving illuminationpattern 105 is a uniform, or Lambertian pattern with a reduced powerlevel 106 in the sidelobes of the radiation pattern, combined with atruncated Lambertian pattern with increased power level 104, in theregion between first limiting angle 102 and second limiting angle 103.Each of the above described radiation patterns may also be achieved inelevation, or ψ-plane as well as the horizontal ⊖-plane as shown. Inaddition, the profiles in the ψ-plane and the ⊖-plane do not have tomatch, and the radiation profile may be shaped as necessary to providethe exact coverage required by the particular application.

Diffuser holder 24 also serves to couple a portion of the output laserlight pulse into optical cable 28 via fiber optic connector 27 andconnector ferrule 17, also shown in FIGS. 2 and 2A. The polished endface of fiber optic connector ferrule 17 positioned at a right angle tooptical axis OA 25 receives adequate energy from each outgoing laserpulse to provide Infrared Optical Receiver 4 with the necessary opticalsignal to establish a stable and accurate zero time reference, and toderive clock timing information as well. Connector body 28 is typicallyan “FC” type, with a threaded body ideal for inserting into a matinghole, counterbored and tapped, in Diffuser holder 24. The polishedendface of connector ferrule 17 of the “FC” style fiber optic connector27 exposes the active region of an optical fiber which serves as anoptical pickup, sampling the outgoing laser illuminating pulses as theypass through Diffuser 26 along optical axis line OA 25. This opticalsample of the outgoing laser illuminating pulse is provided to theInfrared Optical Receiver 4 as the Automatic Range Correction (ARC)signal via optical cable assembly 28 comprised of cable 28 andconnectors 27 as one of the connecting elements of interconnections (7of FIG. 1) between Laser Transmitter 1 and Infrared Optical Receiver 4.

When the laser pulse starts to rise, the light output at 1064 nmincident on the back facet mirror 29 of the Nd:YAG laser impinges uponoptical pickup 31. Optical pickup 31 may be an exposed ferrule of anoptical connector similar to optical connector 27. Optical Pickup 31 isthen connected to the Laser Transmitter Controller 35 through connection32 which is a fiber optic connector in the case where Optical Pickup 31is an exposed ferrule of a fiber optic connector. Inside the LaserTransmitter Controller 35, a photodetector diode terminates thefiberoptic cable and converts the signal thereon into an electricalsignal which is then passed through an amplifier and threshold voltagedetector as described previously herein. This detected, amplified, andthreshold tested signal indicates the onset of a laser pulse in LaserGain Media 19, and is labeled as FLASH_DET. The FLASH_DET signal isnormally used by Laser Controller 35 to terminate the current supply toDiode Pump Lasers 18 and 33 to avoid a double pulse output from theLaser Transmitter 1.

Alternatively, Optical Pickup 31 may be a photodiode positioned tointercept a portion of the light pulse incident upon the rear facetmirror 29 of the Nd:YAG laser, in which case connection 32 is anelectrical connection comprised of stranded or solid wire, or in afurther embodiment, a flex circuit connecting electrically to the LaserTransmitter Controller 35.

Laser Transmitter Controller 35 also monitors the magnitude and durationof the currents supplied to the Diode Pump Lasers 18, and 33 and reportsthese values to any supervising external controller as part of the LaserTransmitter 1 status data as Operating Parameters. Additionally, LaserTransmitter Controller 35 monitors temperature data for the Diode PumpLasers 18 and 33 as well as Laser gain Media 19, and reports these lasertransmitter status data as Laser Temperatures (3). Laser TransmitterController 35 also provides the FLASH_DET signal as a system statusoutput to any external supervising controller. Based on the values ofthe temperature data for each of the Diode Pump Lasers 18 and 33, andthe Laser Gain Media 19, Laser Transmitter Controller 35 calculates andapplies a forward or a reverse current to TE Cooler 36, causing it tocool or heat the common heatsink 42 in FIG. 2A. By cooling or heatingthe common heatsink 42, Diode Pump Lasers 18 and 33 are kept within safezones and at the proper operating wavelength for optimal absorption byLaser gain Media 19. Heatsink 42 also serves as the alignment mechanismfor all the constituent optical components of the Laser Transmitter 1 ofFIG. 2A, enabling a “monoblock” design. Heatsink 42 may be a flatceramic substrate of Beryllium Oxide, Aluminum Nitride, or of a metallicmaterial such as Kovar. The advantages of the “monoblock” design includebetter alignment stability, ease of manufacturing, and thermalperformance. The substrate material which heatsink 42 is formed from isnominally matched thermally to the expansion coefficient of the LaserGain Media 19, and may contain raised features 44, V-grooves, U-shapedchannels, or etched or mechanically polished features (not shown) to aidin the initial alignment of the constituent optical elements of LaserTransmitter 1 during assembly. The various optical elements of the LaserTransmitter 1 are mechanically retained by the common heatsink 42 usingepoxy or adhesive, or by a deformable heat conducting compressible tapeor mechanical spring elements, acting in concert with opticalconfinement cover 43. Similar mechanical constructs act to retain theoptical elements of the end pumped Laser Transmitter 1 of FIGS. 3, 3Aand the thin disc Laser Transmitter 1 of FIGS. 4, 4A.

The difficulty in accurately predicting the onset of a laser pulse hasbeen previously described. This difficulty is a result of the variableelectrical-to-optical efficiencies of Diode Pump Lasers 18 and 33, theirinitial wavelength match to the Laser Gain Media 19 absorption curve,their wavelength drift versus the Laser Gain Media 19 absorption curve,spatial variations of the Laser Gain Media 19, and spatial variations ofthe Q-Switch 20 absorption and bleaching rates, for example. In order toovercome these temporal variations affecting the onset of lasing inLaser Gain Media 19, Laser Transmitter Controller 35 creates a Firecurrent pulse which turns on Q-Switch Trigger Laser 30. The Fire currentpulse is appropriately timed to occur slightly before the onset oflasing would occur naturally from the bleaching of Q-Switch 20. Ineffect, Q-Switch Trigger Laser 30 creates the bleaching of Q-Switch 20prematurely, albeit in a preferred location within Q-Switch 20, and at acontrolled and repeatable interval calculated and determined by LaserTransmitter Controller 35. Q-Switch Trigger Laser 30 is typically asemiconductor laser and may have a wavelength anywhere in the absorptionband of Q-Switch 20. The wavelength of Q-Switch Trigger Laser 30 ischosen in a tradeoff between available power, ease of coupling, andinitial cost, and may be anywhere in the range of 750-1570 nm. Theentire Laser Transmitter 1 assembly is covered by a white anodizedaluminum optical confinement cover 43 in FIG. 2A, which serves the dualpurpose of increasing overall efficiency by reflecting stray light backinto the Laser Gain Media 19, and preventing any potentially unsafelevels of near IR laser light to be viewed directly by an operator.

The lowest order propagating mode (fundamental cavity mode) is the axialmode directly along optical axis OA 25. This mode has the lowesttemporal dispersion, and would be most desirable from a systemoperations point of view, if it could be maintained uninterruptiblyduring an output laser pulse from Laser Transmitter 1. Higher ordermodes are those which propagate in the outer reaches of Laser Gain Media19 in a parallel configuration (zero degree angle) to the fundamentalmode, and those modes bound by the cavity which propagate at a non-zeroangle to optical axis OA 25. It is precisely these higher order modeswhich create temporal dispersion of the Laser Transmitter 1 pulseoutput. In the case of the parallel rays, non-uniformities in theQ-Switch 20 may cause modal partitioning noise, as the Q-Switch 20bleaches laterally from a local absorption minima in the Q-Switch 20material, to a local energy maxima caused by Diode Pump Lasers 18, 33,or in the reverse order. Off-axis modes may exit the Laser Gain Media 19at higher angles, and therefore exhibit path-length based dispersion dueto transit time within Laser Gain Media 19, as well as dispersion basedon spherical aberrations associated with optical elements in the opticalpath. The Side Pumped Laser Transmitter 1 of FIG. 2 exhibits a number ofissues related to the preferential excitation of these higher ordermodes.

Because the cross section of Laser Gain Media 19 is a rectangular orcircular rod, Diode Pump Laser 18, 33 light is absorbed near the outer(lateral or radial) surfaces of Laser Gain Media 19, producing exactlythe kind of elevated energy levels which result in bleaching of theQ-Switch 20 at local energy maxima along one or both of the long sidesof Laser Gain Media 19. By adding rod lenses 39, 40 as shown in FIG. 2A,the energy of Diode Pump Lasers 18 and 33 can be more uniformlydistributed, and focused internal to the Laser Gain Media 19, preferablyon or near Optical Axis 25. Rod lenses 39 and 40 increase the images ofthe small spot sizes of Diode Pump Lasers 18, 33 making a more uniformillumination pattern, reducing the potential for local energy maxima,and together create a convergence of Diode Pump Laser 18, 33 light on ornear Optical Axis OA 25 near the center of rod shaped Laser Gain Media19. The net effect is an improvement of temporal pulse shapes.

Further improvements to the temporal pulse shapes of the Side PumpedLaser Transmitter 1 of FIG. 2 involve other methods of preferentiallypumping or exciting the lowest order laser propagating modes, andsuppressing the propagation of higher order modes unintentionallyexcited by the Diode Pump Laser 18, 33 coupling scheme. Shown first inFIG. 2C is an improvement to Q-Switch 20 which creates an absorptionprofile increasing with radial distance from Optical Axis OA 25.Q-Switch 20 is shown in a circular rod configuration, as opposed to therectangular rod configuration of the preferred embodiment, though theabsorption profiling method may be used to adapt either geometry withbeneficial effect. Q-Switch 20 is sliced from a Chromium dopedcylindrical Nd:YAG rod which has a baseline as-grown doping level 108 of1%. The rod is then spin-coated with a liquid bearing a highconcentration of Chromium and placed in a diffusion furnace. TheChromium doped Nd:YAG rod is then subjected to a thermal profiletailored to create the doping profile 96 shown in FIG. 2C. This GainGuiding Q-Switch 20 of FIG. 2C overcomes some of the limitationsassociated with the Side Pumped Laser Transmitter 1 of FIG. 2, 2Adescribed above. The doping profile 96 is generally an exponential,though the shape is not critical, and the baseline 1% doping level 108and the peak doping level 107 of 2.5% may be varied within the scope ofthe instant invention. Shown in FIG. 2D is a second improvement to theSide Pumped Laser Transmitter 1 of FIG. 2, 2A designed to suppresshigher order modes unintentionally excited by the Diode Pump Laser 18,33 coupling scheme. Back Facet Mirror 29 has been modified to have aconverging surface 97, with steeper sides and a substantially flatsurface in a central region close to Optical Axis OA 25. The convergingsurface 97 of Back Facet Mirror 29 may be conical, spherical, or of aparabolic shape without deviating from the intent or teaching of theinstant invention. Similarly, the interior mirror surface 99 of OutputCoupler 23 may be shaped convergently to suppress the higher orderpropagating modes unintentionally excited by the coupling scheme ofDiode Pump Lasers 18 and 33. Shown at the right of FIG. 2D is themodified Output Coupler 23 with convergent interior mirror surface 99showing a central region 98 which is substantially flat. The shape ofOutput Coupler 23 interior mirror surface 99 may be conical, parabolic,or spherical, or aspherical without deviating from the intent orteachings of the instant invention.

FIG. 3 shows an alternate embodiment of Laser Transmitter 1. In thisversion of Laser Transmitter 1, the diode pump lasers have beenconsolidated into a single Diode Pump Laser module 37 which is coupledto the back facet mirror 29 of Laser Gain Media 19 through Index GuidingTaper 38. The back facet mirror 29 has been modified to have a greaterthan 99% transmissivity at the 808 nm wavelength of Diode Pump Lasermodule 37, while maintaining a greater than 99% reflectivity at the 1064nm wavelength of Nd:YAG Laser Gain Media 19. This manner of pumping theLaser Gain Media 19 is capable of producing a number of benefits.Because the interaction length of the Diode Pump Laser light output withthe Laser Gain Media 19 is much greater than in the side pumpedconfiguration of FIGS. 2, 2A, there is less of a need to stabilize thewavelength of Diode Pump Laser 37. The interaction length for the pumplight with the lasing media can be as great as 50 mm in the end pumpedconfiguration, as opposed to less than 5 mm in the side pumpedconfiguration of FIG. 2, 2A. Therefore, the elimination ofThermoelectric Cooler 36 in FIGS. 2, 2A is possible, and at a greatbenefit to the overall design, because of the inefficiencies associatedwith TE Cooler 36. The peak efficiency of the TE Cooler 36 isapproximately 40%, and in most conditions is closer to 25%. Thisefficiency rating means the Laser Transmitter 1 of FIG. 2 must dissipateat minimum 2.5 times the power dissipated by the Diode Pump Lasers 18and 33 of FIG. 2, a power requirement which taxes the system powersupply (not shown) and limits the operating temperature profile of theassembled Laser Transmitter 1 of FIGS. 2, 2A to a typical range of 0-40degrees Centigrade. By eliminating the TE Cooler 36 of FIG. 2, the EndPumped Laser Transmitter of FIG. 3 power consumption is dramaticallyreduced, and the Laser Transmitter 1 shown in layout form in FIG. 3A canthen be operated over a temperature range of −20 to 70 degreesCentigrade.

Diode Pump Laser 37 is an assemblage of a number of linear diode arrays,normally attached to a common ceramic or metallic heatsink 45 in FIG.3A. The output light from Diode Pump Laser module 37 is coupled to theLaser Gain Media 19 back facet mirror 29 via Index Guiding Taper 38,which collects all the available pump diode light and guides it to theback facet mirror 29 of Laser Gain Media 19 while simultaneouslytapering the mode field diameter in two dimensions. The reduced modefield diameter facilitates better temporal waveshapes in the LaserTransmitter 1, by preferentially exciting the lower order cavity modesof Laser Gain Media 19, which modes exhibit lower temporal dispersionthan the higher order cavity modes preferentially excited in the sidepumped configuration of FIGS. 2 and 2A. A refractive optic may be usedin lieu of Index Guiding Taper 38 with similar beneficial effects,though with a slightly less efficient optical coupling.

FIG. 3A shows the physical layout of the end pumped variant of LaserTransmitter 1 shown in block diagram form in FIG. 3. Notably, the formfactor is longer and narrower compared to the side pumped LaserTransmitter 1 of FIG. 2A. Index Guiding Taper 38 can be seen to have ataper in both vertical and horizontal planes, while many other featurescommon to the basic structure of FIGS. 2 and 2A remain unchanged. Boththe Gain Guiding Q-Switch of FIG. 2C and the Mode Suppressing Mirrors ofFIG. 2D may be used to improve and enhance the spatial and temporalcharacteristics of the End Pumped Laser Transmitter 1 of FIGS. 3, 3A.

FIG. 4 shows a further development of Laser Transmitter 1 in blockdiagram form. The Side Pumped Disc Laser Transmitter 1 of FIG. 4 isshown in layout form in FIG. 4A for key elements of the design and willbe described in tandem with FIG. 4 herein. Examining FIG. 4, it can beseen the complexity of the design is reduced dramatically. Diode PumpLasers 18 and 33 are now complemented by an additional six (6) diodepump lasers 59-64, on three new axes as can be seen in FIG. 4A for atotal of eight (8). A smaller number of diode pump lasers will alsowork, down to a minimum of two, but six to eight has been determined tobe an optimum number for the purposes of this design. Shown in thediagram of FIG. 4A is an octagonally shaped thin disc Laser Gain Media50, but other disc shapes may be used with similar benefits such as:hexagonal, circular, elliptical, or any regular polygon with a number ofsides. The intersection of the optical pump energy for these eight DiodePump Lasers 18, 33, and 59-64 of FIGS. 4, 4A, creates a local maximum ofexcited states in a central region (56 in FIG. 4A) at the center of theLaser Gain Media 50 which coincides with front facet mirror region 49and rear facet region 57. The side pumped disc Laser Transmitter 1embodiment of FIGS. 4, 4A, utilizes a different Laser Gain Media 50comprised of Erbium doped Phosphate glass, or Er:Glass. The lasingwavelength of the Er:Glass gain media is centered around 1540 nm, whichis at or near the desired wavelength for eye safety, propagation, anddetector sensitivity. Because the wavelength of Laser Gain Media 50 isat the fundamental wavelength desired, there is no need for a secondarywavelength conversion, and thus the OPO 22 of FIGS. 2, 2A, and 3, 3A iseliminated. The elimination of the OPO (22 in FIG. 2) is of considerablebenefit because the efficiency of the wavelength conversion process mayonly be 50%, resulting in a large power penalty to both the side pumpedand end pumped rectangular rod laser designs of FIGS. 2 and 3. Tradedoff against the improved efficiency possible due to the elimination ofOPO 22 is the reduction in optical efficiency of the Erbium glass LaserGain Media 50 of the Disc Laser Transmitter 1 versus the Neodymium:YAGLaser Gain Media 19 of the side or end pumped Laser Transmitter 1 ofFIGS. 2, 2A, 3, and 3A. Therefore, though the overallelectrical-to-optical efficiency may be little changed, the benefits ofsingle mode output and a self aligning structure may be considereddecisive improvements of the geometry of Side Pumped Disc LaserTransmitter 1 of FIGS. 4, 4A.

The material composition for Q-Switch 20 may also be altered to bettermatch the operating wavelength of 1540 nm because the Q-Switch 20 is nowoperating in an environment where it is working with the fundamentaltransmission wavelength of 1540 nm of the Laser Gain Media 50, ratherthan the basic wavelength of 1064 nm of the Nd:YAG media of previousFIGS. 2 and 3. Shown schematically in FIG. 4 is the back facet mirror 58with rear facet region 57. Light entering the cavity from the radiallyarranged Pump Laser Diodes 18, 33, 59, 60, 61, 62, 63, and 64 laterallyalong axis B-B in FIG. 4A is absorbed by Laser Gain Media 50. The lightmay be conditioned by an optional lensing element such as rod lens 55illustrated in profile in FIG. 4A in the Right Side View. Lensing andwaveguide apparatus are interchangeable in this role, both capable ofconditioning the light from the Diode Pump Lasers 18, 33, and 59-64, tocreate a desired illumination profile, and either may be used. As energybuilds up in the Er:Glass Lasing Media 50, some premature stimulatedemission may take place in the outer regions where back facet 58 issloped as shown clearly in FIG. 4A Right Side View. Back facet mirror 58is reflective at both the 976 nm wavelength of Diode Pump Lasers 18, 33,and 59-64, and at the desired lasing wavelength of 1540 nm, and may be amultilayer dielectric coating, or may also be a simple reflectivemetallic coating, such as aluminum, silver or gold. The sloping backfacet mirror 58 gently guides any premature stimulated emissions towardsthe central region 56 shown in FIG. 4A. These premature stimulatedemissions might otherwise build up into self sustaining lasing modes,which could theoretically rob the central region 56 of some of theenergy designed to be radiated through the partially reflective centralregion front facet mirror 49. At the center of sloping back facet mirror58, a flat mirror region 57 creates an ideal lasing cavity structuretogether with Laser Gain Media 50, Q-Switch slab 20, and partiallyreflective front facet mirror 49 disposed over central region 56. Whilethe back facet mirror 58 of the preferred embodiment is linearlytapered, other convergent shapes are anticipated which will have similarbenefits to the design. Convergent mirror shapes which are parabolic,spherical, or aspherical are anticipated, and are expected to showimproved performance. The sloped, or conically tapered back facet mirror58 has been chosen for simplicity of fabrication and assembly. Partiallyreflective front facet mirror 49 in the central region has a reflectancein the range of 20-98%, depending on the desired pulse power andduration. The remainder of the front facet mirror 47 outside the centralregion is of the highest reflectivity as is practical, and is typicallygreater than 99% reflective.

In operation, Diode Pump Lasers 18, 33, and 59-64 receive a currentpulse from Laser Transmitter Controller 35 and begin to pump Laser GainMedia 50 with 976 nm laser light. Because the thin disc Laser Gain Media50 typically has a diameter of between 2 and 10 centimeters, theinteraction length between the Diode Pump Lasers 18, 33, and 59-64 isgreat enough so the wavelength of the Diode Pump Lasers 18, 33, and59-64 does not need to be closely controlled. This long interactionlength means there is no need to provide active temperature control toDiode Pump Lasers 18, 33 and 59-64. The 976 nm pump light is containedwithin the Laser Gain Media 50 by intracavity mirror 46 which is highlyreflective at 976 nm, typically greater than 99% reflective at 976 nm.Intracavity mirror 46 is also as highly transmissive as possible at the1540 nm wavelength of Laser Gain Media 50, typically 99% transmissive.As energy levels rise within the Er:Glass Laser Gain Media 50, somestimulated emissions begin to occur, and may cause bleaching of Q-Switchslab 20 in outer regions of the device, close to the Diode Pump Lasers,and distant from the central region 56. Because of the sloping backfacet 58 in these outer regions, no self-sustaining laser modes at the1540 nm wavelength can arise. The 1540 nm light emissions in these outerregions are gently guided toward central region 56 where they sum withthe light from the other pump sources at 976 nm and with any otherpremature 1540 nm light to create a region with the highest density ofexcited states within the Laser Gain Media 50. Because the mirrorsurface 57 in the central region of back facet mirror 58 is flat, aself-sustaining lasing mode may arise once Q-Switch 20 bleaches in theregion described by the dashed lines in FIG. 4A Right Side View, thecentral region 56. Because of the ultrashort cavity length in the rangeof 150-450 micrometers, only a single wavelength will be selected whichoverlaps the gain profile of Er:Glass doped Laser Gain Media 50, andthus will be established a single longitudinal mode (ray 65 shownexiting the laser at 49) in the central region 56 of the Side PumpedDisc Laser shown in FIGS. 4, 4A. Once a single longitudinal mode 65along optical axis OA 25 is set up, a single transverse mode 66, showinga nearly Gaussian profile will also be set up, illustrated at the topright of FIG. 4A. Side Pumped Disc Laser Transmitter 1 may also make useof the radially tapered Q-Switch 20 absorption profile described inassociation with FIG. 2C, in order to reduce the probability of excitinghigher order modes in Laser Gain Media 50, which could potentially robthe device of power in the desired central region 56 by siphoning offenergy to feed parasitic modes outside this central region 56.

Fiber optic connector 31 is in contact with an opening in the commonheatsink 53 and penetrates the back facet mirror surface at or near thecenter of the flat mirror surface 57, substantially aligned with opticalaxis OA 25. Heatsink 53 may be of ceramic or metallic materials,designed to remove heat and retain the optical elements in alignment.The polished end of the ferrule projecting from the “FC” connector body31 of the preferred embodiment may act in one of two capacities. First,it may serve to couple an optical sample of the output laser pulse backto the Laser Controller 35 through fiber optic cable 32 as an ARC signalas described above in connection with FIGS. 2, 2A. Second, it may serveas the initiation point for “seeding” the laser mode at the center ofthe side pumped disc Laser Transmitter 1 along optical axis OA 25.Seeding the laser mode along optical axis OA 25 may be accomplished byusing the fiber optic cable 32 and fiber optic connector 31 to couple alower power laser output from a semiconductor laser (not shown)triggered by a Fire pulse from Laser Controller 35. The Diffuser 26 andDiffuser holder 24 may still be used in association with fiber opticconnector 27 and ferrule 17 to produce an ARC signal from the front ofthe laser as previously described. However, in some cases, it may beuseful to have two fiber optic cables and connectors 31, 32 positionedat the rear of the laser close to optical axis OA 25 as shown, using oneto create an optical ARC signal, and the other to preferentially seedthe fundamental axial mode 65 of the side pumped disc Laser Transmitter1 of FIGS. 4, 4A. It is well known in the art if a single longitudinallasing mode with a single transverse Gaussian power spectral densityprofile 66 is obtained, excellent temporal pulse behavior will be theresult. As such, this side pumped disc Laser Transmitter 1 is capable ofproviding the greatest of benefits to the flash ladar of the instantinvention. The ensuing discussion of Infrared Optical Receiver 4 willillustrate the benefits of the various improvements described above inthe discussion of FIGS. 2, 2A, 3, 3A, 4, and 4A.

FIG. 5 shows the infrared optical receiver 4 in block form. Receiveoptics 2 comprised of a fixed lens 88 and optional motorized zoom lens87 intercepts a portion of the transmitted laser pulse reflected fromthe scene in the field of view as Received Light 15. This light signalis conditioned and directed to fall as a focal plane array input 67 onreceive sensor front end 72 which is comprised of a 128×128 focal planearray of APD optical detectors 73 in the preferred embodiment. APD FocalPlane Array 73 sits atop Readout IC 76 which has a Front End section 74.Front End section 74 consists of an array of individual electricalcircuits termed unit cells. Each unit cell has an input connected to oneof the pixels of the APD Focal Plane Array 73, and typically, each pixelof APD Focal Plane Array is connected to an input of a unit cell. Eachunit cell typically contains a low noise amplifier with variable gain, athreshold detecting circuit, a timing circuit, and a sampling circuit.The operation of the unit cell circuitry has been detailed in prior artpatents U.S. Pat. No. 6,133,989, U.S. Pat. No. 5,696,577, U.S. Pat. No.6,414,746, and U.S. Pat. No. 6,362,482 B1, which are incorporated hereinby reference. The outputs of the individual unit cells are selected bythe System Controller 3, and the data read out sequentially. The datafor each pixel read out by System Controller 3 typically consists of ananalog value representing the intensity of the light received by thepixel (or a digital representation thereof), the time of arrival of thelight pulse received by the pixel, and in some cases, a series of analogsamples (or digital representations thereof) representing the shape ofthe received light pulse.

Drive electronics 75 produce a ZOOM_DRIVE output capable of driving themotorized zoom lens 87 of the receive optics 2 and transmits this drivesignal to the zoom lens via bidirectional electrical connections 85.Drive electronics 75 also reads the shaft encoders or optical encodersindicating position of the zoom lens via bidirectional electricalconnections 85 and produces a RX_ZOOM_STATUS signal for communicatingInfrared Optical Receiver 4 status signals to System Controller 3through bidirectional electrical connection 84. Infrared OpticalReceiver 4 status signals 79 are bundled with Infrared Optical Receiver4 control signals 78 and bidirectional electrical connections 84 andshown together as bidirectional electrical connections 10 betweenInfrared Optical Receiver 4 and System Controller 3 in FIG. 1.Bidirectional electrical connections 10 also serve to connect theZOOM_CTRL signal from System Controller 3 to Drive Electronics 75.

Drive Electronics 75 also receives control commands from SystemController 3 intended for Receive Sensor Front End 72 including ZERO_REFtiming information, transmitted pulse shape coefficients, system clock,calibration factors, etc., necessary for operation of the Receive SensorFront End 72 via bidirectional electrical connections 10. These signalsare passed between Drive Electronics 75 and Receive Sensor Front End 72via bidirectional electrical connections 86. The Drive Electronics 75also generate the APD Focal Plane Array 73 bias voltage, and providesthis voltage to Receive Sensor Front End 72. Drive Electronics 75 alsoreceives numerous status signals from Receive Sensor Front End 72 viabidirectional electrical connections 86. Drive Electronics 75 alsocontrols the timing of the transfer and conversion of raw data from theReceive Sensor Front End 72 to Output Electronics 80. These raw data aretransferred via electrical connections 77 by controlling OutputElectronics 80 through electrical connections 83 while simultaneouslycontrolling Receive Sensor Front End 72 via bidirectional electricalconnections 86. Portions of the Readout IC (ROIC) responsible for globalfunctions of the ROIC 76, including the logic circuits which control theselection of the row and column of the ROIC Front End 74 individualpixel output amplifiers, also reside within Drive Electronics 75 mountedon printed circuit board assembly 81.

Receive Sensor Front End 72 provides range and intensity data to theOutput Electronics 80 via electrical connection 77 when polled at theend of a laser pulse transmit cycle. A typical frame rate of 30 Hzallows for 33 mS between laser transmit pulses, which means there isample time for the internal range gate to time out, and for the OutputElectronics 80 of Infrared Optical Receiver 4 to readout the raw rangeand intensity data from the Receive Sensor Front End 72, make itscalculations, and transmit these calculated or reduced data via itselectrical output 82 which is bundled together with other InfraredOptical Receiver 4 outputs as electrical connections 14 in FIG. 1. Givena maximum range of 10,000 feet, the range gate can close after 20microseconds (representing a two-way trip), and no further reflectedpulse should be expected. The Output Electronics 80 makes calculationsbased on the raw analog samples of the received light pulses toaccurately fit the centroid of the transmitted laser pulse, or a leadingedge of the transmitted pulse, or some kind of a least squares orexponential curve fit of the transmitted illuminating pulse shape,allowing for a more precise estimate of time of arrival of the reflectedlaser pulse signal 15, and therefore a more precise measurement of therange datum indicated by the reflected laser pulse 15. These pulse shapefitting calculations are done for each pixel of the 16,384 pixels of the128×128 focal plane array (FPA) of the preferred embodiment. The OutputElectronics 80 also adjusts analog values of reflected pulse amplitudefor any DC offsets or gain variance before passing these signals to aninternal analog to digital converter. The corrected and adjusted digitalrepresentations of range and intensity are then transmitted viaelectrical connections 14 either to an internal Image Processor 5, ordirectly to an external display or image processing function.

Output Electronics 80 also receives calibration data from SystemController 3 related to the offset and gain of the track and holdamplifiers driving the input to the analog to digital converter (ADC) ofOutput Electronics 80 via signal line 78 and returns status informationto the system controller 3 via signal line 79. Both signal lines 78 and79 are bundled with bidirectional electrical connections 10 betweeninfrared optical receiver 4 and system controller 3.

It is important to note the effect of the improvements to the LaserTransmitter 1 pulse timing and pulse shape as they affect the distancemeasuring capacity and accuracy of Infrared Optical Receiver 4. Theestimation of the time of arrival of a reflected light pulse returnedfrom the scene in the field of view of the flash ladar camera isnegatively affected by transmitted laser pulses which have poorconformance to a regular Gaussian, exponential, or polynomialmathematical representation. Poor conformance of a transmitted laserilluminating pulse to a regular mathematical function can create timinguncertainties, because the received light pulses are converted into aseries of samples and processed to extract timing data by means of adata reduction. The reduction of the received pulses is an attempt tofit each return pulse shape to a given characteristic transmit pulseshape, typically using a polynomial, series, rational function,centroid, Gaussian, exponential, or some novel combination from thislist, which fits the sampled curve with a number of terms to the chosenfunction, usually employing less than nine (9) terms. Therefore, theimprovements to Laser Transmitter 1 pulse shape which reduce outputpulse-to-pulse timing jitter, eliminate splitting of the transmit pulseshape, and/or improve conformance of the transmitted pulse shape to aGaussian characteristic, improve the distance accuracy of the InfraredOptical Receiver 4 by improving the certainty of the time of arrival ofthe reflected light pulses.

The ARC system is a further attempt to improve the accuracy of the flashlaser radar of the instant invention. The optical sample of the LaserTransmitter 1 output transmitted by the ARC signal cable (28 in FIG. 2,or 32 in FIG. 4A) is provided to the Infrared Optical Receiver 4 and tooptical coupler 68 as shown in FIG. 5, where it is split into two equalpower level optical signals 69 and 71. The optical signal 71 which isfed to APD Focal Plane Array 73 is delayed by fiber optic delay line 70,which is a coil of bare optical fiber between 50-100 meters in length.Both the delayed 71 and undelayed 69 optical signals are physicallycoupled to the input of two side-by-side pixels at a corner of the APDFocal Plane Array 73 by an ARC pixel coupling structure as shown in FIG.6. By sending an undelayed optical sample 69 of the transmit pulsethrough a sacrificial pixel of the APD Focal Plane Array (FPA) 73 andassociated ROIC Front End amplifier, the time delays common to each ofthe individual APD detectors in the FPA 73 and their associated lownoise amplifiers contained in ROIC Front End 74 can be duplicated andthen nulled out by globally subtracting the time delay measured on thepixel which is illuminated by the ARC signal 69 as an initial offsetadjustment. This effectively improves the quality of the zero timereference, which is a basis for all of the time delay measurements forall of the pixels of the entire APD FPA 73, and therefore improves theaccuracy of all of these range measurements.

By using the delayed branch of the ARC signal 71, a calibration of thetiming clock which is the global reference oscillator for all of thecounters associated with each pixel of the APD focal plane array is madepossible. Major portions of the timing clock may be embedded in ROIC 76.This timing clock may operate independently, or may be slaved to thesystem clock provided by System Controller 3. Calibration of the timingclock allows for enhanced range accuracy. The nominal clock frequency of430 MHz is counted in a 15 bit counter which gives a maximum round tripdelay of 76.2 microseconds, or a one way delay of 38.1 microseconds, oran effective maximum range of less than 11,430 meters. The accuracy ofthe on board clock may be calibrated against the known delaydifferential between the undelayed ARC signal 69, and the delayed ARCsignal 71. If the embedded differential delay 70 is 50 meters, and ismeasured during a calibration sequence using a precision timingreference such as a high speed oscilloscope, as having a time delay of163 nanoseconds, the on board clock at 430 MHz should register a countof 70.09. Because this is not a whole number, a phase comparator will beable to indicate to a voltage controlled oscillator to adjust the phaseof the clock by 0.09 of a cycle over the nominal count of 70 for thegiven chip time of 2.3256 nanoseconds associated with the nominal 430MHz clock frequency. A simpler solution is to mathematically scale allthe results of the distance measurements by the fraction of 70.09/70,and to leave the clock as a fixed, and therefore more accurate timingreference, stabilized internally or by a thermally isolated crystal ordielectric resonator. Both these solutions are anticipated by theinstant invention with similar beneficial effects.

Referring to FIG. 6 which shows the physical layout of the ARC signalcoupling to the selected APD Focal Plane Array 73 pixels, the two fibers69 and 71 are coupled side by side into GRIN lens 89 which is attachedto positioning substrate 90. Positioning substrate 90 is held inoptically aligned position by L-shaped projection 92 attached to baseplate 93. Positioning substrate 90 is fitted with ARC mirror surface 91which directs the light from the two adjacent optical fibers 69 and 71onto adjacent pixels in the 128×128 APD Focal Plane Array 73 which sitsatop Readout IC (76 and 74). Positioning substrate 90 may be a stainlesssteel flat with ARC mirror surface 91 precision machined into thesurface, or bent upwards from a flat stainless steel sheet. Otherreflective materials may be used for positioning substrate 90 and ARCreflective mirror surface 91 which perform the same functions, such ascertain types of glass, metal coated ceramics, metallized moldedplastics, etc., without changing the nature or positive effects of thedescribed structure.

FIG. 6A shows an alternative embodiment of the ARC Pixel Couplingstructure described in FIG. 6 above. In this case, the major changes arethe elimination of positioning substrate 90, ARC mirror surface 91, andGRIN lens 89. Projection 92 has been modified from the L-shape of FIG. 6to a straight shape with a V-grooved surface feature 94 which capturesoptical fibers 69 and 71, respectively, and holds them in opticalalignment and in close proximity with the selected ARC pixels of APDFocal Plane Array 73. For the purposes of visual clarity, optical fibers69 and 71 are shown in FIG. 6A illuminating ARC pixels which are notadjacent, but it is possible for optical fibers 69 and 71 to be held inthe same projection 92 which would have two V-grooved surface features94 side-by-side, or on some integer multiple of the pixel-to-pixel pitchof the APD Focal Plane Array 73.

FIG. 6B shows a further alternative embodiment of the ARC Pixel Couplingstructure described in FIG. 6 above. In this case, the design usespositioning substrate 90 as in FIG. 6, and an ARC mirror surface 95which is angle polished onto the end of optical fibers 69 and 71 andwhich directs the ARC optical signals onto the selected pixels of APDFocal Plane Array 73. Optical fibers 69 and 71 are either epoxied topositioning substrate 90, or embedded into positioning substrate 90 andthen secured in position. Positioning substrate 90 may be arigid/flexible optical fiber assembly commonly known in the art asrigid/flex, flexible optical backplane, or flex-fiber. Projection 92 hasbeen modified from the L-shape of FIG. 6 to a straight shape wherein thetop surface provides a location and support for the optical alignment ofoptical fibers 69 and 71 to the desired pixels.

Although the invention of the flash laser radar (flash ladar) system hasbeen specified in terms of preferred and alternative embodiments, it isintended the invention shall be described by the following claims andtheir equivalents.

The invention claimed is:
 1. A flash laser radar having a field of view,the flash laser radar comprising: a pulsed laser transmitter having anoptical axis and a pulsed laser light output, and a diffusing opticadapted to illuminate a scene in the field of view with a pattern oflight intensity; the pulsed laser transmitter comprising a solid stategain media interposed between two mirrors, and coupled to a Q-switch,and said solid state gain media optically pumped by at least one laserdiode, and said at least one laser diode electrically driven by anelectrical current issuing from a laser controller, a zero timereference generator generating a time zero output and signal whichindicates the emission of pulsed laser light output, and the time zerooutput terminating the electrical current issuing from said lasercontroller; receive optics boresighted along the optical axis, thereceive optics adapted to collect and condition an incident opticalsignal of pulsed laser light reflected from the scene in the field ofview; an optical receiver with a light detecting focal plane arrayhaving a plurality of light detecting pixels, the light detecting focalplane array disposed to intercept said collected and conditionedincident optical signal from the receive optics; and wherein eachindividual pixel in the light detecting focal plane array receives apixellated portion of the incident optical signal of pulsed laser lightreflected from the scene in the field of view; and each individual pixelis adapted to convert incident pulsed laser light into an opto-electricpulse signal; a system controller having a laser pulse initiation outputconnected to the laser controller of said pulsed laser transmitter, andsaid laser pulse initiation output generating a signal which initiatesthe electrical current issuing from said laser controller, and saidsystem controller also adapted to control the operations of the opticalreceiver; a plurality of unit cell electrical circuits, each unit cellelectrical circuit connected to an individual pixel of the lightdetecting focal plane array; and each unit cell electrical circuitadapted to detect an opto-electric pulse and having a detected pulseoutput; a plurality of timing circuits, each timing circuit connected tothe time zero output and adapted to initiate timing upon a transition ofthe time zero output, and each timing circuit also connected to adetected pulse output of a unit cell electrical circuit, and each timingcircuit adapted to stop timing upon a transition of the detected pulseoutput, and said timing circuit adapted to produce a timing output valueproportional to the elapsed time between the zero time reference and adetected pulse output; and an optical sampler positioned in the outputpath of the pulsed laser transmitter, the optical sampler adapted tocollect a portion of the aforesaid pulsed laser light output; whereinthe optical sampler is connected via an optical pathway to at least oneindividual pixel of the light detecting focal plane array.
 2. The flashlaser radar of claim 1 wherein the optical sampler is opticallyconnected to the at least one individual pixel of said light detectingfocal plane array via an optical waveguide.
 3. The flash laser radar ofclaim 2 wherein the optical waveguide is an optical fiber.
 4. The flashlaser radar of claim 1 wherein the optical sampler is an endface of anoptical fiber.
 5. The flash laser radar of claim 1 wherein the solidstate gain media is a rod shape and said solid state gain media has across section selected from the group consisting of: square, hexagonal,octagonal, circular, rectangular, and elliptical.
 6. The flash laserradar of claim 1 wherein the solid state gain media is a neodymium: YAGcrystal.
 7. The flash laser radar of claim 1 wherein the at least onelaser diode is optically coupled to the solid state gain media throughan optical waveguide.
 8. The flash laser radar of claim 1 wherein the atleast one laser diode is optically coupled to the solid state gain mediathrough a lens.
 9. The flash laser radar of claim 1 wherein the Q-switchis optically triggered by a light pulse from a semiconductor laseroptically coupled to the Q-switch, said light pulse initiated by a lasertransmitter controller.
 10. The flash laser radar of claim 1 wherein theQ-switch has a non uniform bleaching threshold, which increases withradial distance from a central optical axis.
 11. The flash laser radarof claim 1 wherein the Q-switch has a non uniform doping profile whichincreases with radial distance from a central optical axis.
 12. Theflash laser radar of claim 1 wherein at least one laser diode isdisposed so as to have an optical output directed substantially alongthe optical axis of said state gain media and optically coupled to saidsolid state gain media, thereby end pumping the solid state gain media.13. The flash laser radar of claim 12 wherein the optical output of atleast one laser diode is optically coupled to an end face of said solidstate gain media through an optical waveguide.
 14. The flash laser radarof claim 12 wherein the optical output of at least one laser diode isoptically coupled to an end face of said laser gain media through arefractive optic.
 15. The flash laser radar of claim 1 wherein thepulsed laser transmitter is a disc laser comprised of at least thefollowing elements optically coupled in a substantially lineararrangement along an optical axis, with the elements being, in order: aback facet mirror optically coupled to a thin disc laser gain mediawhich is then optically coupled to an intracavity mirror, which is thenoptically coupled to a Q-switch which is then optically coupled to afront facet mirror.
 16. The flash laser radar of claim 15 wherein thethin disc laser gain media is selected from the group consisting of:erbium doped glass and erbium doped phosphate glass.
 17. The flash laserradar of claim 15 wherein a portion of the back facet mirror is aconvergent mirror.
 18. The flash laser radar of claim 15 wherein thethin disc laser gain media is less than 500 wavelengths thick at thelasing wavelength.
 19. The flash laser radar of claim 15 wherein thediameter of the thin disc laser gain media is greater than 1000 timesthe lasing wavelength.
 20. The flash laser radar of claim 15 wherein thethin disc laser gain media has a cross sectional profile selected fromthe group consisting of: a regular polygon and a circle.
 21. The flashlaser radar of claim 15 wherein the thin disc laser gain media isoptically pumped from a plurality of sides by a plurality of laserdiodes positioned in a plane perpendicular to the optical axis of thepulsed laser transmitter.
 22. The flash laser radar of claim 21 whereinat least one of the laser diodes is optically coupled to the thin disclaser gain media through a lens.
 23. The flash laser radar of claim 21wherein at least one of the laser diodes is optically coupled to thethin disc laser gain media through a waveguide.
 24. The flash laserradar of claim 15 wherein the Q-switch is optically triggered by a lightpulse from a semiconductor laser optically coupled to the Q-switch, saidlight pulse initiated by a laser transmitter controller.
 25. The flashlaser radar of claim 15 wherein the Q-switch has a non uniform bleachingthreshold, increasing with radial distance from a central optical axis.26. The flash laser radar of claim 15 wherein the Q-switch has a nonuniform doping profile, the non uniform doping profile increasing withradial distance from a central optical axis.
 27. The flash laser radarof claim 1 wherein said at least one optical sampler is opticallyconnected to at least one individual pixel of said light detecting focalplane array via optical pathway consisting of an optical waveguide, alens, and a mirror surface.
 28. The flash laser radar of claim 1 whereinsaid at least one optical sampler is optically connected to at least oneindividual pixel of said light detecting focal plane array via opticalpathway consisting of an optical waveguide, and wherein said opticalwaveguide has an angle polished endface for directing light towards atleast one individual pixel.
 29. A flash laser radar having a field ofview, the flash laser radar comprising: a pulsed laser transmitterhaving an optical axis and a pulsed laser light output, and a diffusingoptic adapted to illuminate a scene in the field of view with a patternof light intensity; the pulsed laser transmitter comprising a solidstate gain media interposed between two mirrors, and coupled to aQ-switch, and said solid state gain media optically pumped by at leastone laser diode, and said at least one laser diode electrically drivenby an electrical current issuing from a laser controller, a zero timereference generator generating a time zero output and signal whichindicates the emission of pulsed laser light output, and the time zerooutput terminating the electrical current issuing from said lasercontroller; receive optics boresighted along the optical axis, thereceive optics adapted to collect and condition an incident opticalsignal of pulsed laser light reflected from the scene in the field ofview; an optical receiver with a light detecting focal plane arrayhaving a plurality of light detecting pixels, the light detecting focalplane array disposed to intercept said collected and conditionedincident optical signal from the receive optics; and wherein eachindividual pixel in the light detecting focal plane array receives apixellated portion of the incident optical signal of pulsed laser lightreflected from the scene in the field of view; and each individual pixelis adapted to convert incident pulsed laser light into an opto-electricpulse signal; a system controller having a laser pulse initiation outputconnected to the laser controller of said pulsed laser transmitter, andsaid laser pulse initiation output generating a signal which initiatesthe electrical current issuing from said laser controller, and saidsystem controller also adapted to control the operations of the opticalreceiver; a readout integrated circuit having a plurality of unit cellelectrical circuits, each unit cell electrical circuit connected to anindividual pixel of the light detecting focal plane array; and each unitcell electrical circuit adapted to detect an opto-electric pulse andhaving a detected pulse output; a plurality of timing circuits, and eachtiming circuit connected to the time zero output and adapted to initiatetiming upon a transition of the time zero output, and each timingcircuit also connected to a detected pulse output of a unit cellelectrical circuit, and each timing circuit adapted to stop timing upona transition of the detected pulse output, and said timing circuitadapted to produce a timing output value proportional to the elapsedtime between the zero time reference and a detected pulse output; andsaid readout integrated circuit adapted to output the timing outputvalues, and an optical sampler positioned in the output path of thepulsed laser transmitter, the optical sampler adapted to collect aportion of the aforesaid pulsed laser light output; wherein the opticalsampler is connected via an optical pathway to at least one referencepixel of the light detecting focal plane array, and the zero timereference generator connected to said reference pixel, and adapted toproduce the time zero output.
 30. The flash laser radar of claim 29wherein the optical sampler is optically connected to the at least oneindividual pixel of said light detecting focal plane array via anoptical waveguide.
 31. The flash laser radar of claim 30 wherein thesaid optical waveguide is an optical fiber.
 32. The flash laser radar ofclaim 29 wherein the optical sampler is an endface of an optical fiber.33. The flash laser radar of claim 29 wherein the solid state gain mediais a rod shape and said solid state gain media has a cross sectionselected from the group consisting of: square, hexagonal, octagonal,circular, rectangular, and elliptical.
 34. The flash laser radar ofclaim 29 wherein the solid state gain media is a neodymium: YAG crystal.35. The flash laser radar of claim 29 wherein the Q-switch is opticallytriggered by a light pulse from a semiconductor laser optically coupledto the Q-switch, said light pulse initiated by a laser transmittercontroller.
 36. The flash laser radar of claim 29 wherein the Q-switchhas a non uniform bleaching threshold, which increases with radialdistance from a central optical axis.
 37. The flash laser radar of claim29 wherein the Q-switch has a non uniform doping profile which increaseswith radial distance from a central optical axis.
 38. The flash laserradar of claim 29 wherein the optical output of at least one laser diodeis optically coupled to an end face of said solid state gain mediathrough an optical waveguide.
 39. The flash laser radar of claim 29wherein the at least one laser diode is optically coupled to the solidstate gain media through a lens.
 40. The flash laser radar of claim 29further comprising at least one laser diode positioned to have anoptical output directed substantially along the optical axis of saidstate gain media and optically coupled to said solid state gain media,thereby end pumping the solid state gain media.
 41. The flash laserradar of claim 29 wherein the pulsed laser transmitter is a disc lasercomprised of at least the following elements optically coupled in asubstantially linear arrangement along an optical axis, with theelements being, in order: a back facet mirror optically coupled to athin disc laser gain media which is then optically coupled to anintracavity mirror, which is then optically coupled to a Q-switch whichis then optically coupled to a front facet mirror.
 42. The flash laserradar of claim 41 wherein the thin disc laser gain media is selectedfrom the group consisting of: erbium doped glass and erbium dopedphosphate glass.
 43. The flash laser radar of claim 41 wherein the thindisc laser gain media is less than 500 wavelengths thick at the lasingwavelength.
 44. The flash laser radar of claim 41 wherein the diameterof the thin disc laser gain media is greater than 1000 times the lasingwavelength.
 45. The flash laser radar of claim 41 wherein the thin disclaser gain media has a cross sectional profile selected from the set of:a regular polygon and a circle.
 46. The flash laser radar of claim 41wherein the thin disc laser gain media is optically pumped from aplurality of sides by a plurality of laser diodes positioned in a planeperpendicular to the optical axis of the pulsed laser transmitter. 47.The flash laser radar of claim 41 wherein at least one of the laserdiodes is optically coupled to the thin disc laser gain media through alens.
 48. The flash laser radar of claim 41 wherein at least one of thelaser diodes is optically coupled to the thin disc laser gain mediathrough a waveguide.
 49. The flash laser radar of claim 41 wherein theQ-switch has a non-uniform bleaching threshold, the bleaching thresholdincreasing with radial distance from a central optical axis.
 50. Theflash laser radar of claim 41 wherein the Q-switch has a non uniformdoping profile, the doping profile increasing with radial distance froma central optical axis.